In order to improve the performance and amplitude of a giant magnetostrictive transducer, this paper establishes a mathematical model of saturation magnetostrictive coefficient and prestress based on the mechanism of the effect of prestress on a giant magnetostrictive. We put forth the concept of magnetostrictive sensitivity and proposed a theoretical model of magnetostrictive sensitivity, external magnetic field intensity and prestress. In order to maximize the amplitude of the ultrasonic transducer, we selected the optimal prestress with the criteria of the optimal average magnetostrictive sensitivity. The experimental results showed that average magnetostrictive sensitivity exists optimal value with increasing prestress, and maximum ultrasonic amplitude was obtained under a certain magnetic field, thereby verifying the validity of the model of magnetostrictive sensitivity and the feasibility of the optimal prestress method. The proposed model of optimal prestress has guiding significance for the choice of prestress, design, and application of a giant magnetostrictive transducer.